Farmers must contend with another herbicide-resistant weed

The rate of glyphosate increases from left to right, from the normal rate to 32 times the normal rate. the weed is Italian ryegrass, which has grown resistant to glyphosate. Photo by Daniel Stephenson

News Release Distributed 04/14/14

ALEXANDRIA, La. – LSU AgCenter weed scientists confirmed that farmers will now have to contend with another herbicide-resistant weed, Italian ryegrass.

Daniel Stephenson, LSU AgCenter weed scientist, said seed from two fields in Tensas Parish was used to grow the plant to confirm resistance to glyphosate.

“It is pretty well widespread in the northeast part of the state and has been found in St. Landry Parish as well,” he said.

The weed has the potential to rob a corn plant of 75 percent of its yield. It also poses a problem for later-planted crops such as cotton and soybeans because clumps of dead Italian ryegrass can interfere with planting and seedling growth.

A fall application of an alternative herbicide, Dual Magnum, is the best way to control it, he said, but the regular burn-down herbicide treatment of glyphosate and 2,4-D won’t kill it. Dual Magnum also has the added benefit of suppressing the weed henbit, he said.

The Mississippi State University protocol for adoption of a glyphosate-resistant Italian ryegrass management program was developed in response to the weed’s discovery in that state two years ago.

Donnie Miller, AgCenter weed scientist, said Italian ryegrass, even if it’s not herbicide-resistant, has been a problem for row crops. “It’s going to suck moisture out of the seedbed,” Miller said.

Because it is a winter annual, the weed could cause problems for wheat, Miller said. “It competes directly with the crop.”

In addition, Italian ryegrass seed can be comingled at harvest with wheat grain, causing quality issues when a crop is sold, Miller said.

Stephenson said the plant emerges in September and October. Then its emergence slows in December, but it emerges again in January.

He said farmers who neglect to treat the weed in the fall will pay more in the form of additional herbicide treatments in the spring before planting.

Stephenson said the MSU guidelines indicate that two applications of paraquat at a high rate can control it now and stop seed production, but this would be considered a salvage treatment.

It typically is found on roadsides, in ditches and on the edges of fields, he said, and farmers working their land unknowingly spread the seed across a field.

But Miller said Italian ryegrass is not as prolific of a seed producer as Palmer amaranth, which also has developed herbicide resistance, so farmers can see the gradual spread of the weed across the field.

“It’s like watching a hurricane versus a tornado,” Miller said. “With Italian ryegrass, you can see it coming.”

In addition to Italian ryegrass and Palmer amaranth, weeds recently found to have developed glyphosate herbicide resistance include johnsongrass, rice flatsedge and barnyardgrass.